There are conventional adjusting means for adjusting a biasing force of biasing means for biasing the backrest toward an upright position as below:
(A) At a winding end of the coil spring for biasing the backrest toward an upright position, the nut which engages with the thread of the adjusting rod is turned by the handle to change initial load of the coil spring in Patent Literature 1.
(B) A plurality of rubber torsion springs is arranged along the pivot shaft turning with the backrest, and biasing force of the rubber torsion spring selectively acts on the pivot shaft in Patent Literature 2.
(C) On the transmitting path of a biasing force, changing a position where the biasing force acts applies strength and weakness to the biasing force to be transmitted in Patent Literatures 3 and 4.